This invention relates to a method of sinking a caisson made of reinforced concrete or the like material into an underground position.
In its broader sense, the method of the invention can be effectively utilized in embedding a tank or the like underground, in forming a vertical shaft into the ground, and so on.
In general, caissons of the type referred to include open and pneumatic caissons, piles for vertical shafts and so on. Such caissons are sunk, for example, by excavating the ground with an excavator, such as a clam bucket to make a relatively deep vertical shaft, placing a tapered bottom edge of the caisson along a peripheral wall that is prepared to be easily collapsible and causing the peripheral wall of the shaft to collapse immediately below the tapered bottom edge of the caisson to sink the caisson. This caisson sinking method has had problems when the ground contains heterogeneous layers or foreign substances, the caisson tends to be tilted, thereby reducing safety and working efficiency.
In an attempt to eliminate the above problem, a method has been suggested in Japanese Patent Publication No. 43532/1983 by T. Nagai et al., wherein many sand or gravel columns are preliminarily constructed vertically through the ground at a site of embedding the caisson. The tapered bottom edge of the caisson is placed on these columns, and the sand or gravel forming the columns and material between the respective sand or gravel columns are removed to sink the caisson gradually. This method reduces the undesirable tilting of the caisson first described above. However, there still remain many problems with the method disclosed by Nagai, et al. so that, even when one tries to sink the caisson with its normal attitude maintained, the safety and working efficiency cannot be improved to a satisfactory extent.
Especially in the caisson sinking arrangement of the Japanese patent publication by Nagai et al., a plurality of circular shafts are made along an annular contour as a whole, and sand or gravel is cast into the respective shafts. Zones still remain between the shafts that may contain heterogeneous materials or foreign matter, which may cause the caisson to be tilted. Yet, as the respective shafts are made to be much larger than the thickness of the tapered bottom edge of the caisson for the purpose of securing a sand or gravel removal zone, the larger inner diameter of the shafts requires the troublesome preliminary step and an increased amount of sand or gravel. This Japanese patent publication is considered to be suitable for use with a large-diametered caisson, but the construction of the plurality of shafts to highly accurately align them along an annular line requires a high degree of control of the excavating positions of the respective shafts.